The present invention relates to an ion source of a residual gas analyzer which can be used in ultrahigh vacuum regions, and more specifically to an ultrasensitive ion source of the hot-cathode electron impact type, which is small in size, which enables the easy removal of gases, and which permits only a very small energy dispersion in the obtained ionic current.
Hot-cathode electron-impact ion sources are often used in mass analyzers, etc., because of their high sensitivity and high stability. In recent years, vacuum techniques have developed rapidly, and ultrahigh vacuum conditions of 10.sup.-6 Pa (.perspectiveto.10.sup.-8 Torr) can be obtained easily. In this vacuum region, the quality of the vacuum, i.e., the analysis of residual gases, is of importance. Therefore, a mass analyzer which can employ a hot-cathode electron-impact type of ion source plays an important role as a residual gas analyzer. To explain this in more detail, the gases remaining in an ultrahigh vacuum have mass numbers smaller than that of carbon dioxide, which is 44. Therefore, a mass analyzer capable of measuring mass numbers of between 50 to 100 will suffice for this purpose. A quadruple electrode type of mass analyzer is often used, but it does not enable any reduction in resolution, although it does allow some dispersion of the energy of the generated ions.
When such a device is to be used under ultrahigh vacuum conditions of less than 10-8 Torr, however, the gases emitted from the ion source impede the correct analysis of the residual gases. To analyze residual gases in the ultrahigh vacuum region, therefore, a quadruple electrode type of mass analyzer provided with a BA gauge type of electrode-impact ion source with a cage-like grid anode is chiefly used, since it has a relatively high sensitivity and permits the easy removal of gases. Even with this BA gauge ion source with its high sensitivity and easy gas removal, however, its sensitivity is about 3.times.10.sup.-4 A/Torr at the most with an electron current of between 2 to 5 mA. Under ultrahigh vacuum conditions of less than 10.sup.-8 Torr, therefore, the obtained ionic current is at most: (.perspectiveto.3.times.10.sup.-4 A/Torr).times.(10.sup.-8 Torr)=.perspectiveto.3.times.10.sup.-12 A. Therefore, even if an attempt is made to analyze residual gases with a resolution of about 10%, the current obtained is less than .perspectiveto.3.times.10.sup.-13 A, and dc amplification alone is not sufficient for analyzing residual gases under vacuum conditions of less than 10.sup.-8 Torr. In order to analyze residual gases under vacuum conditions of less than 10.sup.-8 Torr, therefore, it has been proposed to amplify the ionic current by between 10.sup.5 to 10.sup.6 using a secondary electron multiplier device. However, not only is a gas analyzer equipped with a currently-available secondary electron multiplier device relatively large in size and expensive, the secondary electron multiplier device is prone to large changes with time, so that the analyzer has a poor reliability, and requires a cumbersome handling operation.
These problems result from the fact that even with a BA gauge highly-sensitive ion source, the utilization efficiency of the generated ions is as low as about 1/100 to 1/10. This stems from a defect of the BA gauge type of ion source in that the energy of the ions generated in the cage-like grid anode is dispersed to a large extent (.apprxeq.50 eV). Even with a quadruple-electrode mass analyzer that allows some degree of energy dispersion, the incident energy of the ions must be suppressed to less than about 10 eV when the length of the poles of quadruple electrodes is less than 10 cm, so that the ions generated by the ion source are not all utilized. The construction and function of a BA gauge type of ion source will be described below with reference to a conventional example that is shown in the drawings. FIG. 1 is a section through a BA gauge type of ion source. Thermoelectrons emitted from a hot-cathode filament 1 are attracted by a cylindrical cage-like anode 2, travel through the cage, are reflected by a repeller electrode 3 on the opposite side, are attracted again by the cage-like anode 2, and travel through the cage repeatedly, to ionize the gas molecules.
The vibrating electrons are eventually captured by the cage-like anode 2. However, the electric current flowing through the hot-cathode filament 1 is controlled by an electronic circuit so that the electronic current obtained through the cage-like anode 2 is always constant. Thus large quantities of cations are generated around the cage-like anode 2. However, the ions generated within the cage-like anode 2 are attracted by the negative electric field of an ion-extraction electrode 4 that is inserted into the cage-like anode 2 through an ion-extraction port formed in the anode 2, so that the ions are emitted from the cage-like anode 2 through the ion-extraction port. The electrons vibrate within the cage-like anode 2, not only in the lateral directions, but also in the vertical direction, so that large quantities of ions are generated even at the portion of ion-extraction port where the potential of the introduced electric field is low. The ions generated on the surface of the anode are far from the ion-extraction port, and are less attracted thereby, but the ions generated in the low-potential region near the ion-extraction port are drawn thereby very efficiently. Therefore, the energy of the ions obtained through the ion-extraction electrode 4 is very dispersed, and is uniformly distributed along the potential gradient of the cage-like anode 2 and the ion-extraction electrode 4. The potential difference between the two electrodes is at least about 80 volts (when the maximum energy of the electrons is 60 eV), and the energy dispersion of the obtained ions is about 50 eV. In a quadruple-electrode mass analyzer, ions of a large energy dispersion that have passed through the ion-extraction electrode 4 must be decelerated to less than 10 eV before reaching an analyzer portion 5. Therefore, the efficiency with which the ionic current is utilized is low. For instance, when the incident ions have an average energy of 10 eV, the energy dispersion is distributed over the whole range between 0 to 20 eV, so that ions of an energy greater than 10 eV pass through the analyzer portion 5 without any mass analysis, reducing the resolution. When ions have a large energy dispersion, it is difficult to converge an ion beam with an electrostatic lens system, and the sensitivity is reduced.